DESCRIPTION

libgps is a service library which supports querying GPS devices; link
it with the linker option -lgps. There are two interfaces supported in
it; one high-level interface that goes through gpsd(1) and is intended
for concurrent use by several applications, and one low-level interface
that speaks directly with the serial or USB device to which the GPS is
attached. This page describes the low-level interface, which gpsd(1)
itself uses. See gpsd(3) for a description of the high-level interface,
which is almost certainly what you want.
Calling gpsd_init() initializes a session structure to hold the data
collected by the GPS.
You may optionally specify a DGPS server, either as a string containing
a server name or a string containining server name followed by a colon
and a port name or number. To specify no DGPS, pass the null pointer.
The second argument must be a context structure. The library will use
it for information that need to be shared between sessions; presently
this includes the leap-second correction and possibly a pointer to a
shared-memory segment used to communicate with the Network Time
Protocol daemon.
After the session structure has been set up, you may modify some of its
members.
gpsd_device
This member should hold the path name of the device.
baudrate
Communication speed in bits per second. For NMEA or SiRF devices,
the library automatically hunts through all plausible baud rates,
stopping on the one where it sees valid packets. By setting this
field you can designate a speed to be tried at the front of the
hunt queue
raw_hook
A hook function to be executed on each NMEA sentence or as it is
read from the GPS. The data from non-NMEA GPSes like the EarthMate
will be translated to an NMEA sentence before being passed to the
hook. Parameters are a pointer to a gps_data structure full of
parsed data, the sentence, the length of the sentence, and a
rawness level.
gpsd_activate() initializes the connection to the GPS.
gpsd_deactivate() closes the connection. These functions are provided
so that long-running programs can release a connection when there is no
activity requiring the GPS, and re-acquire it later.
gpsd_poll() queries the GPS and updates the part of the session
structure that holds position, speed, GPS signal quality, and other
data returned by the GPS. It returns a mask describing which fields
have changed.
gpsd_wrap() ends the session, implicitly performing a
gpsd_deactivate().
The calling application must define one additional function:
gpsd_report(). The library will use this to issue ordinary status
messages. Use first argument of 0 for errors, 1 for ordinary status
messages, and 2 or higher for debugging messages.
The low-level functions do not allocate or free any dynamic storage.
They can thus be used in a long-running application (such as gpsd(8)
itself) with a guarantee that they won´t cause memory leaks.